Transition metals such as titanium are plentiful in earth's crust, occur in abundance in the form of oxides (e.g., as rutile-TiO2 and ilmenite-FeTiO3), and have highly useful properties. Titanium, in particular, is a metal suitable for applications that require a material having a low specific gravity, high relative strength and strength-to-weight ratio, even at high temperatures. For example, titanium metal has been used since the 1950s as a structural material, first in aerospace and defense applications. Subsequently, titanium has been used in chemical applications, to form biomedical prosthesis, and in leisure and sport equipment. In addition, titanium is generally highly resistant to corrosion, and often forms surface layers that are stable to chlorides and acids.
Like many other transition metals, however, titanium is generally considered difficult to process. It is expensive to extract and reduce from its ores, and relatively difficult to fabricate into useful products in view of its high melting point, and oxidation properties. In addition, metal powders having a precisely controlled composition and/or microstructure are typically required in powder metallurgy techniques such as hot isostatic processing. For transition metals such as titanium, known techniques for purification and powder preparation are relatively expensive, particularly if the metal is to be rendered suitable for advanced powder metallurgical manufacturing processes.
Two commercial multi-step titanium extraction processes coexisted until the early 1990s: the Kroll and the Hunter processes. Currently, titanium metal is typically produced by reducing titanium tetrachloride with molten magnesium or sodium metal in a steel batch retort. When TiCl4 (“tickel”) is mixed with the magnesium or sodium metal reducing agent, highly exothermic reactions occur, thereby producing a crude intermediate titanium “sponge.” The sponge typically contains titanium metal as well as intimately mixed contaminants and by-products such as magnesium or sodium chloride, titanium subchlorides, and impurities originally present in the reducing agent. The titanium sponge is then refined to produce titanium ingots for manufacturing use. Sponge refining typically also involves costly processes such as the use of vacuum arc technologies.
Numerous titanium production paths have been proposed, and exemplary paths are listed in Table 1. They generally suffer from different drawbacks. For example, production paths that require chemical reduction of titanium compounds typically involve the formation of intermediate compounds that contain high levels of impurities. Purity, separation, oxidation and other issues associated with intermediate compounds may present technical and economic challenges. In particular, intermediate products formed by chemically reducing titanium halides tend to be highly contaminated with halides. Impurities such as oxides, carbon, and in some instances, nitrides may be formed as well. In addition, plasma thermal reduction of titanium chlorides utilizes heating to extremely high temperatures, and is accordingly very energy intensive. All of these processes are also disadvantageous since they are expensive.
Electrochemical processes also suffer from technical and economic disadvantages. While it is possible to deposit metallic Ti onto an electrode, such deposition typically must be carried out using a molten salt system. These electrochemical processes are typically associated with high energy cost as well as labor costs of removing and stripping the electrode onto which metallic Ti is deposited. Such costs represent substantial economic obstacles in commercializing electrolytic Ti processing techniques. Furthermore, molten salt processes typically require high current densities for high industrial throughputs. However, high current densities tend to favor dendrite growth. As a result, technical issues such as electrical shorts, separation from the melt, and product densification must be addressed in such molten salt processes.
In processes under development based on electrochemical deoxidation of TiO2, for example, the use of molten chloride electrolytes typically containing CaCl2 results in the production of fine Ti powder that is intermixed with the remaining calcium species. If this powder is then washed, a significant amount of surface titanium oxide is formed that must later be removed. Since it is difficult and expensive to remove oxygen below the about 300 ppm level required for most modern uses, the need for further cleaning and purification steps results in significantly increased costs.
TABLE 1PROCESSREDUCING AGENT*RESULTING PRODUCT*Chemical ReductionNaTi + NaClof TiCl4Na and AlCl3TiiAlj + NaClMgTi + MgCl2AlTi or TiiAlj + AlCl3Electrochemicale−Ti + X2Reduction TiXi in amolten salt bathChemical ReductionNaTi + NaFof TiF4MgTi + MgF2AlTi + AlF3Electrochemicale−Ti sponge or powder + F2Reduction of TiF4Chemical ReductionTiTi + TiI2 + I2of TiI4Plasma AssistedH2 → 2HTi + H2OReduction of TiO2CTi + CO (TiOiC)C + NTiN + 2COChemical ReductionCaTi(O) + CaOTiO2of TiO2AlTiAli + (Al2O3), CaO, TiO2*Where X is a halogen such as F, Cl, Br, or I; e− indicates an electrochemical reduction; and i, j represent subscripts with different values.
Similarly, processes based on the reduction of titanium tetrachloride with an alkali or alkaline earth metal such as liquid sodium, e.g., according to the Armstrong et al. process of U.S. Pat. No. 6,409,797 also result in the production of fine titanium powders mixed with byproducts such as NaCl and excess reactants. Typically, such processes require additional means and process steps, e.g., elaborate systems of vacuum distillation and leaching, to provide clean titanium.
Processes utilizing fluidized bed reactors in which TiCl4 is reduced by a gaseous metal such as Mg have also been disclosed. In U.S. Pat. No. 4,877,445 to Okudaira et al., for example, titanium pellets are produced by reducing titanium tetrachloride in vapor form using magnesium or sodium vapor as the reducing agent. However, the Okudaira et al. process requires the injection of reducing agent vapors and continuous operation at high temperature to recover, e.g., MgCl2 as a condensable vapor. Impurities in the vapor reducing agent such as Mg will also appear at least to some degree in the titanium product. In addition, the use of magnesium results in titanium production costs similar to those of the Kroll process.
Once ingots are formed, a number of techniques may be used to produce parts having a complex geometry. For example, ingots may be melted, poured into a mold, cooled, and removed from the mold. Such casting processes are generally unsuited for low volume production runs due the cost of the molds. In addition, it is sometimes difficult to control the microstructure of parts made via casting processes. Alternatively, machining techniques may be used to selectively remove portions of ingots to produce parts of a desired shape. The removed portions of the ingot, of course, represent a source of waste. While powder metallurgy techniques have been developed that allow complex shapes to be formed quickly, titanium metal powders are currently quite expensive. Beside the costs associated with ingot production, powders incur the added costs associated with subsequent alloying and atomizing steps for producing uniform powders from the refined ingot.
Thus, there is a need in the art for technologies useful in lowering the cost associated with the production of high-purity metallic compositions, particularly for transition metals such as titanium and alloys thereof. In addition, there is a need to overcome the problems associated with known processes for producing metallic compositions that involve the production of halide-contaminated intermediate products by providing alternative, economically attractive methods for directly forming high-purity, dry and clean metallic granules, including the direct production of metal alloys, from metal halides. More particularly, it would be very desirable to provide a process for the direct production of titanium and titanium alloys in which there is no need to further clean and purify such metals using subsequent processing steps and wherein the cost is substantially reduced through the use of a cheap, abundant and clean reducing agent.